Tunable Luminescence From a Single Free-Base Porphyrin Molecule By Controlled Access to Optically Active States

Technical Summary: Tunable Luminescence From a Single Free-Base Porphyrin Molecule By Controlled Access to Optically Active States

Problem Statement
Scanning tunneling microscopy-induced luminescence (STML) offers a pathway to probe the optical properties of individual molecules via relaxation processes between many-body states. However, a significant limitation exists for certain intrinsically bright molecules, such as free-base tetrabenzoporphyrins (H2TBP), which remain "dark" in STML experiments on Ag(111) substrates. The authors identify that insufficient charge attachment energies quench the relaxation cascade via optically excited states. Specifically, H2TBP exhibits a smaller hole attachment energy compared to its chemical analog, phthalocyanine (H2Pc), resulting in an emission suppression of approximately 98% on Ag(111). This energy misalignment prevents the necessary transition from the positively charged ground state ($D^+_0$) to the first optically excited singlet state ($S_1$), hindering high-sensitivity applications like vibronic satellite detection or fluorescence mapping.

Methodology
The study employs STML and scanning tunneling spectroscopy (STS) on individual H2TBP and H2Pc molecules decoupled from Ag(111) and Ag(110) substrates by 3–5 monolayer (ML) insulating NaCl islands. The experimental approach involves:

1. Energy Level Characterization: Recording STS spectra to determine ion resonances and transport gaps, followed by fitting these spectra using a polaron model. This model accounts for the structural relaxation energy ($\lambda$) of the NaCl decoupling layer upon molecular charging, which significantly shifts the apparent ion resonance energies observed in STS.
2. Electrostatic Tuning: Investigating two methods to align energy levels:
   • Tip-Induced Gating: Utilizing the voltage drop across the double-barrier tunnel junction (DBTJ) to shift the molecular electrochemical potential ($\Delta E$).
   • Substrate Work Function Control: Comparing measurements on Ag(111) ($\Phi \approx 4.5$ eV) and Ag(110) ($\Phi \approx 4.1$ eV) to induce a static shift in molecular ion resonances.
3. Theoretical Modeling: Developing a rate equation model to simulate transition rates ($S_0 \to D^+_0 \to S_1$) based on the polaron-derived energy levels and experimental STS data.
4. Spectral Analysis: Performing hyperspectral STML imaging and comparing experimental vibronic structures with Time-Dependent Density Functional Theory (TD-DFT) calculations (B3LYP/def2-TZVP) including Herzberg-Teller (HT) coupling.

Key Results

• Mechanism of Quenching: Polaron modeling reveals that on Ag(111), the true ground-state energy of the positively charged H2TBP ($D^+_0$) is approximately 100 meV below the $S_1$ state energy (1.81 eV). This energetic barrier prevents the $D^+_0 \to S_1$ transition, explaining the observed quenching. In contrast, for H2Pc, the $D^+_0$ state lies ~200 meV above $S_1$, enabling bright emission.
• Limitations of Tip Gating: While applying negative bias voltages shifts the $D^+_0$ state upward, the voltage drop across the junction (estimated at ~10%) is insufficient to fully overcome the energy gap and the resonance broadening caused by NaCl relaxation. Even at high biases (-2.5 V), emission remains quenched by ~94%. The model suggests biases < -5.6 V would be required for optimal alignment, a regime that risks transitioning to field emission and damaging the molecule.
• Success via Substrate Engineering: By depositing H2TBP on Ag(110), the 400 meV reduction in substrate work function shifts the $D^+_0$ energy to 2.25 eV. This provides ~300 meV of excess energy relative to the $S_1$ state, comparable to the favorable alignment seen in H2Pc. Consequently, the emission intensity of H2TBP on Ag(110) increases by nearly two orders of magnitude, and a secondary emission line ($S_2$) at 2.15 eV becomes detectable.
• Vibronic and Spatial Mapping: The enhanced quantum yield on Ag(110) enabled high-resolution fluorescence mapping. The data revealed well-resolved vibronic sidebands for both $S_1$ and $S_2$ states. TD-DFT calculations incorporating Herzberg-Teller coupling showed strong agreement with experimental spectra, particularly for $S_1$. Spatial maps indicated a dual-mirror symmetry for $S_1$ (attributed to fast tautomerization averaging) and a distinct nodal plane with 4-fold symmetry for $S_2$.

Significance and Claims
The paper claims to demonstrate that the "dark" nature of single H2TBP molecules in STML is not an intrinsic property of the molecule but a result of unfavorable energy level alignment relative to the substrate and decoupling layer. By controlling the substrate work function, the authors successfully engineered the energy landscape to access optically active states that were previously forbidden.

The significance of this work lies in:

1. Validating the Polaron Model: Confirming that NaCl relaxation energy is a critical factor in determining the effective energy requirements for electroluminescence in STML.
2. Enabling High-Resolution Characterization: Overcoming the quenching barrier allows for the detailed photophysical characterization of free-base porphyrins, including the resolution of electronic and vibronic structures and the mapping of excited-state spatial distributions.
3. Methodological Advancement: Establishing that substrate work function engineering, rather than just tip-induced gating, is a necessary strategy for achieving efficient electrical excitation in systems with large relaxation energies.

The authors conclude that these controlled energy level alignments provide a comprehensive picture of the photophysics of free-base porphyrins, offering a pathway to study their electronic and spin tunability in hybrid structures.